Liquid-crystalline medium

ABSTRACT

Liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I, ##STR1## in which R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted by CN or CF 3  or at least monosubstituted by halogen, it also being possible for one or more CH 2  groups in these radicals to be replaced, in each case independently of one another, by --O--, --S--, ##STR2## --CO--, --CO--O--, O--CO-- or --O--CO--O-- in such a way that O atoms are not linked directly to one another and additionally comprises one or more compounds selected from the group consisting of the general formulae II, III, IV, V and VI defined herein.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid crystals are used, in particular, as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN (twisted nematic) cells, STM (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The most common display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should haverelatively low Viscosity and give short addressing times, low thresholdvoltages and high contrast in the cells.

Furthermore, they should have a suitable mesophase, for example anematic or cholesteric mesophase for the abovementioned cells, atconventional operating temperatures, i.e., in the broadest possiblerange above and below room temperature. Since liquid crystals aregenerally used in the form of mixtures of a plurality of components, itis important that the components are readily miscible with one another.Further properties, such as the electrical conductivity, the dielectricanisotropy and the optical anisotropy, must satisfy differentrequirements depending on the cell type and area of application. Forexample, materials for cells having a twisted nematic structure shouldhave positive dielectric anisotropy and low electrical conductivity.

For example, media of large positive dielectric anisotropy, broadnematic phases, relatively low birefringence, very high resistivity,good UV and temperature stability and low vapour pressure are desiredfor matrix liquid-crystal displays having integrated nonlinear elementsfor switching individual pixels (MLC displays).

Matrix liquid-crystal displays of this type are known. Examples ofnonlinear elements which can be used for individual switching ofindividual pixels are active elements (i.e. transistors). This is thenreferred to as an "active matrix", and a differentiation can be madebetween two types:

1. MOS (metal oxide semiconductor) or other diodes on silicon wafers assubstrate.

2. Thin-film transistors (TFTs) on a glass plate as substrate.

Use of monocrystalline silicon as the substrate material limits thedisplay size, since even modular assembly of the various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A differentiationis made between two technologies: TFTs comprising compoundsemiconductors, such as, for example, CdSe, or TFTs based onpolycrystalline or amorphous silicon. Intensive work is being carriedout worldwide on the latter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, whilst the other glass plate carries the transparentcounterelectrode on the inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fullycolour-compatible image displays, where a mosaic of red, green and bluefilters is arranged in such a way that each filter element is locatedopposite a switchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display containingintegrated nonlinear elements, i.e., in addition to the active matrix,also displays containing passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TV sets) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. In addition to problems with respect to the angledependence of the contrast ad the response times, problems arise in MLCdisplays owing to inadequate resistivity of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of an MLC display drops, and the problem of after-imageelimination can occur. Since the resistivity of the liquid-crystalmixture generally drops over the life of an MLC display owing tointeraction with the internal surfaces of the display, a high (initial)resistance is very important in order to obtain acceptable servicelives. In particular in the case of low-voltage mixtures, it washitherto not possible to achieve very high resistivities. It isfurthermore important that the resistivity increases as little aspossible with increasing temperature and after heating and/or exposureto UV radiation. Also particularly disadvantageous are thelow-temperature properties of the mixtures from the prior art. It isrequired that crystallization and/or smectic phases do not occur, evenat low temperatures, and that the temperature dependence of theviscosity is as low as possible. MLC displays of the prior art thus donot satisfy current requirements.

There thus continues to be a great demand for MLC displays having veryhigh resistivity at the same time as a broad operating temperaturerange, short response times, even at low temperatures, and low thresholdvoltage which do not have these disadvantages or only do so to a reducedextent.

In the case of TX (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

broadened nematic phase range (in particular down to low temperatures),

switchability at extremely low temperatures (outdoor use, automobiles,avionics),

increased stability on exposure to UV radiation (longer life).

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted cells (STN), media are desired which enablegreater multiplexibility and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther extension of the parameter latitude available (clearing point,smectic-nematic transition or melting point, viscosity, dielectricquantities, elastic quantities) is urgently desired. SUMMARY OF THEINVENTION

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the abovementioneddisadvantages, or only do no to a reduced extent, and preferably at thesame time have very high resistivities and low threshold voltages.

It has now been found that this object can be achieved when novel mediaare used in displays.

The invention thus relates to a liquid-crystal-line medium based on amixture of polar compounds having positive dielectric anisotropy,characterized in that it comprises one or more compounds of the generalformula I, ##STR3## in which R is H, an alkyl or alkenyl radical having1 to 15 carbon atoms which is unsubstituted, monosubstituted by CM orCF₃ or at least monosubstituted by halogen, it also being possible forone or more CH₂ groups in these radicals to be replaced, in each caseindependently of one another, by --O--, --S--, ##STR4## --CO--,--CO--O--, --O--CO-- or --O--CO--O-- in such a way that O atoms are notlinked directly to one another.

The compounds of the formula I have a broad range of applications.Depend an the choice of substituents, these compounds can serve as basematerials from which liquid-crystalline media are predominantly cased;however, compounds of the formula I can also be added toliquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or optical anisotropyof a dielectric of this type and/or to optimize its threshold voltageand/or its viscosity.

In the pure state, the compounds of the formula I are colorless and formliquid-crystalline mesophases in a temperature range which is favorablylocated for electro-optical use. They are stable chemically, thermallyand to light.

If R in the compounds of the formula I is an alkyl radical and/or analkoxy radical, this can be straight-chain or branched. It is preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly ispreferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy,butoxy, pentoxy, hexoxy or heptoxy. Furthermore methyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy,octoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy arealso suitable.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl,2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-,6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-,5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkyl radical in which one CH₂ group has been replaced by--CH═CH--, this can be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl,non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-,-4-, -5-, -6-, -7-, -9- or -9-enyl.

If R is an alkyl radical in which one CH₂ group has been replaced by--O-- and one has been replaced by --CO--, these are preferablyadjacent. These thus contain one acyloxy group --CO--O-- or oneoxycabonyl group --O--CO--. Those are preferably straight-chain and have2 to 6 carbon atoms.

Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy,pentanoylozy, hexanoyloxy, acetoxymethyl, propionyloxymethyl,butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxy-ethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-pro-pionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl) ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted --CH═CH-- and an adjacent CH₂ group hasbeen replaced by CO or CO--O or O--CO, this can be straight-chain orbranched. It is preferably straight-chain and has 4 to 13 carbon atoms.Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl,3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl,6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl,9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl,2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl and9-methacryloyloxynonyl.

If R is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CMor CF₃ is in any desired position.

If R is an alkyl or alkenyl radical which is at least monosubstituted byhalogen, this radical is preferably straight-chain and halogen ispreferably F or Cl. In the case of multiple substitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluorine or chlorinesubstituent can be in any desired position, but preferably in theω-position.

Compounds of the formula I which contain wing groups R which aresuitable for polyaddition reactions are suitable for the preparation ofliquid-crystalline polyaddition products.

Compounds of the formula I containing branched wing groups R mayoccasionally be of importance due to better solubility in the customaryliquid-crystalline base materials, but in particular as chiral dopes ifthey are optically active. Smectic compounds of this type are suitableas components for ferroelectric materials.

Compounds of the formula I having S_(A) phases are suitable, forexample, for thermally addressed displays.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy and 1-methylheptoxy.

If R is an alkyl radical in which two or more CH₂ groups have beenreplaced by --O-- and/or --CO--O--, this may be straight-chain orbranched. It is preferably branched and has 3 to 12 carbon atoms.Accordingly, it is, in particular, biscarboxymethyl,2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl,5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl,8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl,bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl,3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl,5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl,7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)octyl,bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl,3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl and5,5-bis(ethoxycarbonyl)hexyl.

The compounds of the formula I are prepared by methods known, per se, asdescribed in the literature (for example in the standard works, such asHoubon-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag,Stuttgart), to be precise under reaction conditions which are known andsuitable for said reactions. Use can also be made here of variants whichare known, per se, but which are not mentioned here in greater detail.

The novel compounds can be prepared, for example, as follows: ##STR5##

The invention also relates to electro-optical displays (in particular,STN or MLC displays having two plane-parallel outer plates which,together with a frame, form a cell, integrated nonlinear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highresistivity located in the call) which contain media of this type, andto the use of these media for electro-optical purposes.

The liquid-crystal mixtures, according to the invention, facilitate asignificant broadening of the parameter latitude available.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy are farsuperior to previous materials from the prior art.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δε was previously only achievable to anunsatisfactory extent. Although systems such as, for example, ZLI-3119have a comparable clearing point and comparatively favorableviscosities, they have, however, a Δε of only +3.

Other mixture systems have comparable viscosities and values of Δε, butonly have clearing points in the region of 60° C.

The liquid-crystal mixtures according to the invention make it possibleto achieve clearing points of above 80°, preferably above 85°,particularly preferably above 90° C., and simultaneously dielectricanisotropy values Δε≧5, preferably ≧7, and a high value for theresistivity while retaining the nematic phase down to -20° C. andpreferably down to -30° C., particularly preferably down to -40° C.,which allows excellent STN and MLC displays to be achieved. Inparticular, the mixtures are characterized by low operating voltages.The TN thresholds are below 2.0 V, preferably below 1.8 V, particularlypreferably <1.6 V.

It goes without saying that a suitable choice of the components of themixtures according to the invention also allows higher clearing points(for example above 110°) to be achieved at higher threshold voltages orlower clearing points to be achieved at lower threshold voltages whileretaining the other advantageous properties. It is likewise possible toobtain mixtures of relatively high Δε and thus relatively low thresholdsif the viscosities are increased by a correspondingly small amount. TheMLC displays according to the invention preferably operate in the firsttransmission minimum of Gooch and Tarry [C. E. Gooch and R. A. Tarry,Electron. Lett. 10, 2-4, 1974; C. H. Gooch and S. A. Tarry, Appl. Phys.,Vol. 8, 1575-1584, 1975]; in this case, a lower dielectric anisotropy inthe second minimum is sufficient in addition to particularly favorableelectro-optical properties, such as, for example, high gradient of thecharacteristic line and low angle dependency of the contrast (GermanPatent 30 22 818) at the name threshold voltage as in an analogousdisplay. This allows significantly higher resistivities to be achievedin the first minimum using the mixtures according to the invention thanusing mixtures containing cyano compounds. A person skilled in the artcan use simple routine methods to produce the birefringence necessaryfor a prespecified layer thickness of the MLC display by a suitablechoice of the individual components and their proportions by weight.

The rotational viscosity at 20° C. is preferably <150 mPa.s,particularly preferably <130 mPa.s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from -20° to +80°.

Measurements of the "capacity holding ratio" (HR) [S. Matsumoto at al.,Liquid Crystals 5, 1320 (1989); K. Kiwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984): G. Weber et al., LicuidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a considerablysmaller decrease in the HR with increasing temperature than do analogousmixtures in which the compounds of the formula I are replaced bycyanophenylcyclohexanes of the formula ##STR6## or esters of the formula##STR7##

In addition, it has been found that novel mixtures comprising compoundsof the formula I have a higher clearing point and a higher Δε thananalagous mixtures comprising cyanophenylcyclohexanes of the aboveformula. Compared with the last-mentioned mixtures, the novel mixturesalso have a lower Δn, which is advantageous for many applications.

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e., they exhibit a significantly smaller decreasein the HR on exposure to UV radiation.

The media according to the invention are preferably based on a plurality(preferably two or more) of compounds of the formula I, i.e., theproportion of these compounds is 5-95%, preferably 10-60% andparticularly preferably in the range 13-50%.

The individual compounds of the formulae I to XII and their sub-formulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

The compound of the formula I is a difluorovinyl ether of the formula Ia##STR8## in which alkyl is an alkyl radical having 1-8 carbon atoms;Medium additionally comprises one or move compounds selected from thegroup consisting of the general formulae II to VI: ##STR9## in which theindividual radicals have the following meanings: R⁰ : n-alkyl, oxaalkyl,fluoroalkyl or alkenyl, in each case having up to 9 carbon atoms,

X⁰ : F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 carbonatoms,

Y¹ and Y² : in each case, independently of one another, H or F,

r: 0 or 1.

The compound of the formula IV is preferably ##STR10## Mediumadditionally one or more compounds selected from the group consisting ofthe general formulae VII to XII: ##STR11## in which R⁰, X⁰, Y¹, and Y²are each, independently of one another, as defined in Claim 2,preferably F, Cl, CF₃, OCF₃, OCHF₂, alkyl, oxaalkyl, fluoroalkyl oralkenyl, in each case having up to 6 carbon atoms.

The medium additionally comprises one or more compounds of the formulaIIa: ##STR12## The proportion of compounds of the formulae I to VItogether is at leant 50% by weight in the total mixture;

The proportion of compounds of the formula I is from 10 to 50% by weightin the total mixture;

The proportion of compounds of the formulae II to VI is from 30 to 70%by weight in the total mixture ##STR13## The medium comprises compoundsof the formulae II, III, IV, V or VI R⁰ is straight-chain alkyl oralkenyl having 2 to 7 carbon atoms

The medium essentially consists of compounds of the formulae I to VI

The medium comprises further compounds, preferably selected from thefollowing group consisting of the general formulae XIII to XVI:##STR14## in which R⁰ and X⁰ are as defined above, and the 1,4-phenylenerings may be substituted by CN, chlorine or fluorine. The 1,4-phenylenering are preferably monosubstituted or polysubstituted by fluorineatoms.

The I: (II+III+IV+V+VI) weight ratio is preferably from 1:10 to 10:1.

Medium essentially consists of compounds selected from the groupconsisting of the general formulae I to XII.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formula II, III, IV, Vand/or VI, results in a significant reduction in the threshold voltageand in low birefringence values, and at the same time broad nematicphases with low smectic-nematic transition temperatures are observed,thus improving the shelf life. Particular preference is given tomixtures which, in addition to one or more compounds of the formula I,comprises one or more compounds of the formula IV, in particularcompounds of the formula IVa in which X⁰ is F or OCF₃. The compounds ofthe formulae I to VI are colorless , stable and readily miscible withone another and with other liquid-crystal materials.

The term "alkyl" covers straight-chain and branched alkyl groups having1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atomsare generally preferred.

The term "alkenyl" covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups. Inparticular, alkenyl groups are C₂ -C₇ -1E-alkenyl, C₄ -C₇ -3E-alkenyl,C₅ -C₇ -4-alkenyl, C₆ -C₇ -5-alkenyl and C₇ -6-alkenyl, in particular C₂-C₇ -1E-alkenyl, C₄ -C₇ -3E-alkenyl and C₅ -C₇ -4-alkenyl. Examples ofpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3pentenyl, 3E-hexenyl,3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl,6-heptenyl and the like. Groups having up to 5 carbon atoms aregenerally preferred.

The term "fluoroalkyl" preferably covers straight-chain groupscontaining terminal fluorine, i. e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term "oxaalkyl" preferably covers straight-chain radicals of theformula C_(n) H_(2n+1) --O--(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably 1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the gradient of the transmissioncharacteristic lines, etc., can be modified as desired. For example,1E-alkenyl radical, 3E-alkenyl radicals, 2E-alkenyloxy radicals and thelike generally give shorter addressing times, improved nematictendencies and a higher ratio between the elastic constants k₃₃ (bend)and k₁₁ (splay) compared with alkyl and alkoxy radicals. 4-Alkenylradicals, 3-alkenyl radicals and the like generally give lower thresholdvoltages and lower values of k₃₃ /k₁₁ compared with alkyl and alkoxyradicals.

A --CH₂ CH₂ -- group generally results in higher values of k₃₃ /k₁₁compared with a simple covalent bond. Higher values of k₃₃ /k₁₁facilitate, for example, flatter transmission characteristic lines in TNcells with a 90° twist (for achieving grey tones) and steepertransmission characteristic lines in STN, SBE and OMI cells (greatermultiplexibility), and vice versa.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI depends substantially on the desired properties, on thechoice of the components of the formulae I, II, III, IV, V and/or VI andon the choice of any other components which may be present. Suitablemixing ratios within the abovementioned range can easily be determinedfrom case to case.

The total amount of compounds of the formulae I to XII in the mixturesaccording to the invention is not crucial. The mixtures may thereforecontain one or more further components in order to optimize variousproperties. However, the effect observed on the addressing times and thethreshold voltage is generally greater the higher the totalconcentration of compounds of the formulae X to XII.

In a particularly preferred embodiment, the media according to theinvention comprises compounds of the formulae II to VI (preferably II,III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂,OCR═CF₂, OCF═CF₂ at OCF₂ --CF₂ H. A favorable synergistic effect withthe compounds of the formula results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI and of the formula IVa are distinguished by their low thresholdvoltages.

The construction of the MLC display according to the invention frompolarizers, electrode base plates and electrodes with surface treatmentcorresponds to the construction which is conventional for displays ofthis type. The term conventional construction here is broadly drawn andalso covers all derivatives and modifications of the MLC display, inparticular also matrix display elements based on poly-Si TFTs or MIMs.

An essential difference between the displays according to the inventionand those customary hitherto based on the twisted nematic cell is,however, the choice of the liquid-crystal parameters in theliquid-crystal layer.

The liquid-crystal mixtures which can be used according to the inventionare prepared in a manner which is conventional, per se. In general, thedesired amount of the component used in the lesser amount is dissolvedin the components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and, after thorough mixing, to remove the solvent again, forexample by distillation.

The dielectrics may also contain other additives known to those skilledin the art and described in the literature. For example, 0-15% ofpleochroic dyes or chiral dopes can be added.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (view angle perpendicularto the plate surface). t_(on) denotes the switch-on time and t_(off) theswitch-off time at an operating voltage corresponding to 2.5 tires thevalue of V₁₀. Δn denotes the optical anisotropy and n_(o) the refractiveindex. Δε denotes the dielectric anisotropy (Δε=ε.sub.| -ε.sub.⊥, whereε.sub.| is the dielectric constant parallel to the longitudinalmolecular axes and ε.sub.⊥ is the dielectric constant perpendicularthereto). The electro-optical data were measured in a TN cell at the 1stminimum (i.e., at a d·Δn value of 0.5) at 20° C., unless expresslystated otherwise. The optical data were measured at 20° C., unlessexpressly stated otherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by acronyms, with thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n) H_(2n+1) and C_(m) H_(2m+1) arestraight-chain alkyl radicals containing n or m carbon atoms,respectively. The coding in Table B is self-evident. In Table A, onlythe acronym for the base structure is given. In individual cases, theacronym for the base structure is followed, separated by a hyphen, by acode for the substituents R¹, R², L¹ and L² :

    ______________________________________                                        Code for R.sup.1,                                                               R.sup.2, L.sup.1, L.sup.2 R.sup.1 R.sup.2 L.sup.1 L.sup.2                   ______________________________________                                        nm       C.sub.n H.sub.2n+1                                                                             C.sub.m H.sub.2m+1                                                                       H   H                                      nOm C.sub.n H.sub.2n+1 OC.sub.m H.sub.2m+1 H H                                nO.m OC.sub.n H.sub.2n+1 C.sub.m H.sub.2m+1 H H                               n C.sub.n H.sub.2n+1 CN H H                                                   nN.F C.sub.n H.sub.2n+1 CN H F                                                nF C.sub.n H.sub.2n+1 F H H                                                   nOF OC.sub.n H.sub.2n+1 F H H                                                 nCl C.sub.n H.sub.2n+1 Cl H H                                                 nF.F C.sub.n H.sub.2n+1 F H F                                                 nF.F.F C.sub.n H.sub.2n+1 F F F                                               nCF.sub.3 C.sub.n H.sub.2n+1 CF.sub.3 H H                                     nOCF.sub.3 C.sub.n H.sub.2n+1 OCF.sub.3 H H                                   nOCF.sub.2 C.sub.n H.sub.2n+1 OCHF.sub.2 H H                                  nS C.sub.n H.sub.2n+1 NCS H H                                                 rVsN C.sub.r H.sub.2r+1 --CH═CH--C.sub.s H.sub.2s -- CN H H                                                       rEsN C.sub.r H.sub.2r+1                                                      --O--C.sub.2 H.sub.2s -- CN H H                                                nAm C.sub.n H.sub.2n+1 COOC.sub.                                             m H.sub.2m+1 H H                       nOCCF.sub.2.F.F C.sub.n H.sub.2n+1 OCH.sub.2 CF.sub.2 H F F                 ______________________________________                                    

Preferred mixture components are show in Tables A and B.

                  TABLE A                                                         ______________________________________                                        1  STR15##                                                                       - PYP                                                                         -                                                                          2  STR16##                                                                       - PYRP                                                                        -                                                                          3  STR17##                                                                       - BCH                                                                         -                                                                          4  STR18##                                                                       - CBC                                                                         -                                                                          5  STR19##                                                                       - CCH                                                                         -                                                                          6  STR20##                                                                       - CCP                                                                         -                                                                          7  STR21##                                                                       - CPTP                                                                        -                                                                          8  STR22##                                                                       - CEPTP                                                                       -                                                                          9  STR23##                                                                       - ECCP                                                                        -                                                                          0  STR24##                                                                       - CECP                                                                        -                                                                          1  STR25##                                                                       - EPCH                                                                        -                                                                          2  STR26##                                                                       - PCH                                                                         -                                                                          3  STR27##                                                                       - PTP                                                                         -                                                                          4  STR28##                                                                       - BECH                                                                        -                                                                          5  STR29##                                                                       - EBCH                                                                        -                                                                          6  STR30##                                                                       - CPC                                                                         -                                                                          7  STR31##                                                                       - B                                                                           -                                                                          8  STR32##                                                                       - FET-nF                                                                      -                                                                          9  STR33##                                                                       - CGG                                                                         -                                                                          0  STR34##                                                                       - CGU                                                                         -                                                                          1  STR35##                                                                       - CFU                                                                      ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        2  STR36##                                                                       - BCH-n.Fm                                                                    -                                                                          3  STR37##                                                                       - CFU-n-F                                                                     -                                                                          4  STR38##                                                                       - Inm                                                                         -                                                                          5  STR39##                                                                       - CC-n-OXF                                                                    -                                                                          6  STR40##                                                                       - CBC-nmF                                                                     -                                                                          7  STR41##                                                                       - ECCP-nm                                                                     -                                                                          8  STR42##                                                                       - CCH-n1EM                                                                    -                                                                          9  STR43##                                                                       - T-nFm                                                                       -                                                                          0  STR44##                                                                       - CGU-n-F                                                                     -                                                                          1  STR45##                                                                       - CDU-n-X                                                                     -                                                                          2  STR46##                                                                       - CGG-n-F                                                                  ______________________________________                                    

The entire disclosure of all applications, patents and publications,cited above and below, and of corresponding German application No. DE19650635.2, filed Dec. 6, 1996 is hereby incorporated by reference.

The examples below are intended to illustrate the invention withoutrepresenting a limitation. Above and below, percentages are percent byweight. All temperatures are given in degrees Celsius. m.p. denotesmelting point, c.p.=clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The data betweenthese symbols represent the transition temperatures. Δn denotes theoptical anisotropy (589 nm, ° C.), and the viscosity (mPa.sec) wasdetermined at 20° C.

EXAMPLE

    ______________________________________                                        Example 1                                                                       CC-3-OXF 8.00 Clearing point [°C.]: +89                                CC-5-OXF 10.00 ΔN [589 nm, 20° C.]: +0.0721                      CCN-34 5.00 Δε [1 kHz, 20° C.]: --                       CCN-35 5.00 V.sub.(10, 0, 20) [V]: 1.80                                       CCP-20 CF3 8.00                                                               CCP-30 CF3 8.00                                                               CCP-40 CF3 8.00                                                               CCP-50 CF3 8.00                                                               CCP-2 F.F.F 12.00                                                             CCP-3 F.F.F 12.00                                                             CCP-5 F.F.F 8.00                                                              ECCP-3 F.F 8.00                                                               Example 2                                                                     CC-3-OXF 5.00 Clearing point [°C.]: +92                                CC-5-OXF 8.00 Δn [589 nm, 20° C.]: +0.0816                       CCP-20 CF3 8.00 Δε [1 kHz, 20° C.]: 7.7                  CCP-30 CF3 8.00 V.sub.(10, 0, 20) [V]: +1.53                                  CCP-40 CF3 8.00                                                               CCP-50 CF3 8.00                                                               CCP-2 F.F.F 12.00                                                             CCP-3 F.F.F 12.00                                                             CCP-5 F.F.F 8.00                                                              CGU-3-F 6.00                                                                  ECCP-3 F.F 6.00                                                               CCP-30CF2.F.F 11.00                                                           Example 3                                                                     CC-3-OXF 8.00 Clearing point [°C.]: +86                                CC-5-OXF 8.00 Δn [589 nm, 20° C.]: +0.0745                       CCH-34 5.00 Δε [1 kHz, 20° C.]: --                       CCH-35 5.00 V.sub.(10, 0, 20) [V]: 1.62                                       CCP-20 CF3 9.00                                                               CCP-30 CF3 8.00                                                               CCP-40 CF3 6.00                                                               CCP-50 CF3 8.00                                                               CCP-2 F.F.F 11.00                                                             CCP-3 F.F.F 12.00                                                             CCP-5 F.F.F 7.00                                                              CGU-3-F 4.00                                                                  ECCP-3 F.F 9.00                                                             ______________________________________                                    

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. Liquid-crystalline medium based on a mixture ofpolar compounds having positive dielectric anisotropy, characterized inthat it comprises one or more compounds of the general formula I,##STR47## in which R is H, an alkyl or alkenyl radical having 1 to 15carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or atleast monosubstituted by halogen, it also being possible for one or moreCH₂ groups in these radicals to be replaced, in each case independentlyof one another, by --O--, --S--, ##STR48## --CO--, --CO--O--, --O--CO--or --O--CO--O-- in such a way that O atoms are not linked directly toone another and additionally comprises one or more compounds selectedfrom the group consisting of the general formulae II, III, IV, V and VI:##STR49## in which the individual radicals have the following meanings:R⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having upto 7 carbons atoms,X⁰ F, Cl, halogenated alkyl, alkenyl or alkoxy having1 to 5 carbon atoms, Y¹ and Y² : in each case, independently of oneanother, H or F, r: 0 or
 1. 2. Medium according to claim 1,characterized in that the proportion of compounds of the formulae I toVI together is at least 50% by weight based on the weight of the totalmixture.
 3. Medium according to claim 1, characterized in that theproportion of compounds of the formula I is from 3 to 80% by weightbased on the weight of the total mixture.
 4. Medium according to claim1, characterized in that the proportion of compounds of the formulae IIto VI is from 20 to 80% by weight in the total mixture.
 5. Mediumaccording to one of claim 1, characterized in that R in the compound ofthe formula I is alkyl having 1 to 8 carbon atoms.
 6. A method of usingthe liquid-crystalline medium according to claim 1 which comprisesincorporating said liquid-crystalline medium in an electro-opticaldevice.
 7. Electro-optical liquid-crystal display containing aliquid-crystalline medium according to claim
 1. 8. Liquid-crystallinemedium based on a mixture of polar compounds having positive dielectricanisotropy, characterized in that it comprises one or more compounds ofthe general formula I ##STR50## in which R is H, an alkyl or alkenylradical having 1 to 15 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen, italso being possible for one or more CH₂ groups in these radicals to bereplaced, in each case independently of one another, by --O--, --S--,##STR51## --CO--, --CO--O--, --O--CO-- or --O--CO--O-- in such a waythat O atoms are not linked directly to one another and additionallycomprises a compound of the formula IIa ##STR52## in which theindividual radicals have the following meanings: R⁰ : n-alkyl, oxaalkyl,fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms,X⁰ F,Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 5 carbon atoms, Y¹and Y² : in each case, independently of one another, H or F.
 9. Mediumaccording to claim 8, characterized in that X⁰ is F or OCF₃, and Y¹ andY² are H or F.